INDIVIDUAL NEUTRON MONITORING IN WORKPLACES WITH
MIXED NEUTRON/PHOTON RADIATION
T. Bolognese-Milsztajn1,, D. Bartlett2, M. Boschung3, M. Coeck4, G. Curzio5, F. d'Errico5, A. Fiechtner3, V. Giusti5, V. Gressier1, J. KylloÈnen6, V. Lacoste1, L. Lindborg6, M. Luszik-Bhadra7, C. Molinos2, G. Pelcot1, M. Reginatto7, H. Schuhmacher7, R. Tanner2, F. Vanhavere4and D. Derdau8
1Institut de Radioprotection et de SuÃrete NucleÂaire, F-92265 Fontenay-aux-Roses, France 2National Radiological Protection Board, Chilton, Didcot OX11 0RQ, UK
3Paul Scherrer Institut, CH-5232 Villigen, Switzerland
4Studiecentrum voor KernenergieÐCentre d'eÂtude de l'eÂnergie nucleÂaire, B-2400 Mol, Belgium 5Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, I-56126 Pisa, Italy 6Statens straÊlskyddsinstitut, SE-171-16 Stockholm, Sweden
7Physikalisch-Technische Bundesanstalt, D-38116 Braunschweig, Germany 8Kernkraftwerk KruÈmmel GmbH, D-21502 Geesthacht, Germany
EVIDOS (`evaluation of individual dosimetry in mixed neutron and photon radiation ®elds') is an European Commission (EC)-sponsored project that aims at a signi®cant improvement of radiation protection dosimetry in mixed neutron/photon ®elds via spectrometric and dosimetric investigations in representative workplaces of the nuclear industry. In particular, new spectrometry methods are developed that provide the energy and direction distribution of the neutron ¯uence from which the reference dosimetric quantities are derived and compared to the readings of dosemeters. The ®nal results of the project will be a comprehensive set of spectrometric and dosimetric data for the workplaces and an analysis of the performance of dosemeters, including novel electronic dosemeters. This paper gives an overview of the project and focuses on the results from measurements performed in calibration ®elds with broad energy distributions (simulated workplace ®elds) and on the ®rst results from workplaces in the nuclear industry, inside a boiling water reactor and around a spent fuel transport cask.
INTRODUCTION
EVIDOS is an European Commission (EC)-sponsored project which, over the course of almost 4 y, will perform an `evaluation of individual dosimetry in mixed neutron and photon radiation ®elds'. One of the primary aims of the project is to establish whether innovative electronic dosemeters with direct reading allow improved determination of personal dose equivalent. This objective requires preliminary investigations in calibration ®elds followed by exten-sive in-®eld measurements in representative work-places of the nuclear industry. Seven European research laboratories with recognised expertise in radiation protection and in detector development are involved in the project.
Reference methods for the determination of per-sonal dose equivalent in workplace ®elds are a central part of this project. Because no current dose-meter gives correct results in all neutron ®elds, refer-ence values have to be derived from spectrometry (with respect to the energy and direction of the radiation) and ¯uence-to-dose equivalent conversion coef®cients.
Various measurement campaigns have already been performed, comprising 2 simulated workplace ®elds and 10 workplace ®elds in the nuclear industry.
While these complex measurements require a detailed analysis, which is still in progress, a selection of the results is already available and is presented here.
MEASUREMENT CAMPAIGNS AND INSTRUMENTS USED
In the early phases of the project, a series of repre-sentative workplaces in the nuclear industry in which workers can receive signi®cant neutron doses were selected. For each workplace, some measurement points were selected in collaboration with the radio-logical safety of®cers of the inspected facilities. These workplaces permit a thorough test of the dosemeters since they differ widely in terms of dose rates, neutron/photon relative intensity, energy/ direction distributions and also temperature, pres-sure, humidity, acoustic noise, vibration, electro-magnetic ®elds, etc.
The area monitors and the personal dosemeters employed in the project are listed in Table 1(1±17). These instruments comprise commercial devices as well as laboratory prototypes developed by some of the partners. Only prototype dosemeters have reached the stage of well-characterised, light-weight battery-operated instrumentation with direct reading(18)are utilised in the project.
Corresponding author: teresa.bolognese@irsn.fr
doi:10.1093/rpd/nch220
For a complete analysis of the results, the esti-mates provided by the individual devices are com-pared with the reference values for dose equivalent quantities determined using neutron and, eventually, photon spectrometry. In the case of neutrons, direc-tion-differential ¯uence distributions are determined along with the energy-differential values. These are needed to correlate isotropic quantities, such as ambient dose equivalent, to non-isotropic ones, such as personal dose equivalent and effective dose. Reference ®eld spectrometry providing energy-differential, direction-integrated ¯uence distributions are primarily performed with a Bonner-sphere system(10) for the entire energy range. The measurements of the double-differential (energy and direction) neutron ¯uence are performed with novel instruments based, respectively, on SDD-spectrometers(11) and Si-diodes(12), and employing dedicated numerical procedures (unfolding methods) for data analysis(16). Information on the direction distribution of the ®elds is also derived from PADC track-etch detectors mounted on different sides of the slab phantom and from analysis of the different detector readings for a novel design of neutron survey instrument (BAE SYSTEMS)(3).
In October 2002, prior to the campaigns in the workplaces, an intercomparison exercise was per-formed at the IRSN simulated workplace facility CANEL/T400(13,14) and at the thermal neutron source SIGMA(15). The purpose of the exercise was
to test some novel instruments and to verify the behaviour of all instruments in a well-established reference ®eld similar to some of those encountered at workplaces. Following the intercomparison, two measurement campaigns have been performed: (1) Campaign number 1 (April 2003) was carried
out in KruÈmmel, Germany, inside a boiling water reactor building and around a storage cask containing spent fuel elements.
(2) Campaign number 2 (June 2003) was carried out in Mol, Belgium, at the BelgonucleÂaire MOX fuel element manufacturing plant and at the SCKCEN VENUS research reactor.
One or two additional campaigns are scheduled for the year 2004: at the BNFL plutonium re-proces-sing plant and thermal oxide re-procesre-proces-sing plant in Sella®eld (UK), and/or inside a pressurised water reactor building and around a transport cask in Ringhals, Sweden.
PRELIMINARY RESULTS FROM MEASUREMENTS IN SIMULATED WORKPLACE FIELDS
The responses of area monitors and personal dose-meters in the SIGMA thermal ®eld and at the CANEL simulated workplace ®eld are reported in Tables 2 and 3, respectively. The personal dosemeters were irradiated on an ISO PMMA
Table 1. Short description of the devices used in the EVIDOS project. Name of
device Short description Commercial (c) orprototype (p)
Sievert instrument Low pressure proportional counters (one of tissue-equivalent plastic and one of graphite) evaluated according to
the variance/covariance technique(1)
p
Berthold LB 6411 Moderator type area monitor c
Harwell N91 Moderator type area monitor c
Wendi-2 Moderator type area monitor with tungsten loaded moderator(2) c
Studsvik 2200's Moderator type area monitor c
BAE SYSTEMS Novel area monitor for H(10) and H
p(10,a) measurements(3) p
Aloka PDM-313 Electronic neutron dosemeter with one silicon detector(4) c
Siemens EPD N Electronic photon/thermal neutron dosemeter with three
silicon detectors(4) c
Siemens EPD N2 Electronic photon/neutrondosemeter with three silicon detectors(4) c
PADC(CR-39) Track-etch detector(chemical electrochemical etching)(5) c
Saphydose-n Electronic neutron dosemeter using a segmentedsilicon diode(6) c
DISN 1.25% (2 mm) Differential reading of two ionisation chambers that are based on
Direct Ion Storage(7) p
DISN 4% (1 mm) Differential reading of two ionisation chambers that arebased on
Direct Ion Storage(7) p
DOS-2002 Electronic photon/neutron dosemeter with one silicon detector(8) p
HpSLAB Superheated drop detector inside a slab phantom(9) p
BTI-PND Fast neutron bubble detector(17) c
PND BDT Combination of fast and thermal neutron
water-®lled phantom at angles of 0, 45 and 75 (angles are stated with respect to the normal on the front face of the phantom). Only the results for 0 are presented here since the reference dose values for 45and 75are not yet available. The readings are those provided directly by the instruments based on their speci®c calibration procedures as established by manufacturers or developers. No attempt has been made yet to harmonise the calibration procedures of the different instruments. The uncertainties given in this paper represent ®rst estimates of standard uncer-tainties although a comprehensive analysis is in progress. It is important to note that the uncer-tainties of the instruments' readings do not include
the contributions from the energy and direction dependence of their response. For area monitors, type A uncertainties are generally negligible whereas for the personal dosemeters they are signi®cant. The type B uncertainties are mostly due to uncertainties from the calibration of the instruments (5%).
The SIGMA facility has been recently charac-terised by comparing MCNP-4C simulations with activation foil and spectrometer results(15). In Table 2, the ambient dose equivalent rates measured by the area monitors are compared with a reference value dH(10)/dt (143.6 5.1) mSv hÿ1. Most of the area monitors appear to underestimate the ambient dose equivalent for thermal neutrons. The personal
Table 3. Personal dose equivalent response of personal dosemeters, RHp Hp(10)measured/Hp(10)referenceand relative standard uncertainties.
Facility
CANEL SIGMA
Name of device RHp u(RHp)/RHp(%) RHp u(RHp)/RHp(%)
Aloka PDM-313 5.1 11 4.3 6 BTI-PND 1.11 14 0.69 15 DISN 1.25% 7.3 15 20 12 DISN 1.25% (2 mm) 0.58 24 1.1 12 DISN 4% 2.3 19 4.2 13 DISN 4% (1 mm) 0.50 23 1.0 12 DOS-2002 1.21 12 1.19 7 HpSLAB 1.21 17 0.99 14 PADC (NRPB) 0.95 13 0.85 9 PADC (PSI) 0.32 15 0.45 31 PND BDT 1.32 13 1.19 10 Saphydose-n 0.82 14 1.07 13 Siemens EPD N 0.16 97 0.36 93 Siemens EPD N2 2.7 12 5.4 7
The references values at CANEL and SIGMA are obtained from Refs (15) and (14), respectively.
Table 2. Ambient dose equivalent response of area monitors, RH* H*(10)measured/H*(10)reference and relative standard uncertainties. Facility CANEL SIGMA Name of device RH u Rÿ H=RH (%) RH u Rÿ H=RH (%) BAE SYSTEMS 1.11 7 0.58 8 Berthold LB 6411 0.85 7 0.88 6 Harwell Leake N91 1.26 7 ± ± SSI Sievert A 1.10 17 1.28 11 SSI Sievert B 0.97 17 0.97 11 Studsvik 2202D 1.26 7 0.75 6
The references values of H(10) at CANEL and SIGMA are obtained from Refs (15) and (14), respectively. The Sievert instrument used the following approximate relation between quality factor, Q, and dose mean lineal energy, yD: Q 0.52 0.28yD.
dose equivalent rates measured by personal dosemeters are compared (Table 3) with a reference value dHp(10)/dt (149.4 5.3) mSv hÿ1(15). Many personal dosemeters show agreement with the reference value within 50%, but one device signi-®cantly underestimates whereas ®ve signisigni-®cantly overestimate.
The CANEL facility(13)simulates, using an accel-erator generated ®eld, neutron spectra representative of some workplaces of the nuclear industry. The neutron ¯uence and ambient dose equivalent H(10) as a function of the energy were determined using MCNP-4C and compared with experimental values derived from spectrometry in the framework of the EUROMET intercomparison(14). The refer-ence value used here is the experimental one, H(10) 122.3 6.1 pSv per monitor count. Because of the complex irradiation conditions, dedi-cated computations of personal dose equivalent Hp(10) are required, which are extremely time con-suming and only preliminary results are available at present(13). Thus, the reading of the personal dose-meters can only be compared with the personal dose equivalent value derived from the EUROMET spec-trometry data(14): H
p(10) 126.5 pSv per monitor count, i.e. assuming a broad and parallel irradiation of the phantom. The relative standard uncertainty is estimated to be 10%. The responses of the area monitors presented in Table 2 show agreement with the reference dose rate within 30%. Of the 14, 8 personal dosemeters, show readings that deviate by less than 50% from the reference values (Table 3).
PRELIMINARY RESULTS FROM
MEASUREMENTS IN WORKPLACE FIELDS OF NUCLEAR INDUSTRY
This paper presents preliminary results of 2 of the 10 workplaces investigated to date. The analysis of direction and energy spectrometry is not complete yet so reference values are available for H(10) but not for Hp(10). The ®rst campaign took place at the nuclear power plant in KruÈmmel (Germany). Meas-urements were performed at four workplaces. Two positions were measured at a spent fuel cask of type NTL11 (`cask midline' and `cask side') and two other positions close to the boiling water reactor inside the reactor building: one was situated in the control rod room underneath the reactor (`reactor SAR') and the other near the top of the reactor outside the safety vessel (`reactor top'). The neutron ¯uence as a function of energy was measured with the IRSN Bonner-sphere spectrometer. At the cask midline, the distribution of ¯uence as a function of energy (Figure 1, dashed line) shows a rather hard spectrum in which 75% of the neutrons have energies between 50 keV and 1 MeV. The ambient dose equivalent rate derived from the spectrum at this point is dH(10)/ dt 141 mSv hÿ1. The energy spectrum at the posi-tion reactor SAR (Figure 1, full line) shows a ther-mal and a fast peak but also a non-negligible epithermal and intermediate energy neutron contri-bution. The ambient dose equivalent rate calculated from this spectrum is dH(10)/dt 46 mSv hÿ1. A complete uncertainty analysis for these results is not
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 0 10 20 30 40 50 60 70 80
.
E Φ E (E ) (c m -2 .s -1 )Neutron Energy (MeV) REAKTOR 1
CASK 1
Figure 1. Relative spectral neutron ¯uence per logarithmic bin-width as a function of energy at KruÈmmel transport cask midline (CASK 1, dashed line) and reactor control rod room (SARÐreactor 1, solid line)
yet possible. Based on previous experience with Bonner-sphere spectrometers, a conservative esti-mate of the relative standard uncertainty is 15%.
Table 4 shows the ambient dose equivalent response of the area monitors at the two measure-ments positions. Except for one case, the deviations from the reference value are <30%.
The Hp(10) reference value is being evaluated using direction and energy spectrometry. This work is still in progress. However, for the cask midline position the radiation ®eld is almost unidirectional and the reference value for Hp(10) can be approxi-mated by dHp(10, 0)/dt 147 mSv hÿ1, derived from Bonner-sphere measurements. A large uncer-tainty of 30% is attributed to this value. The actual value of the Hp(10) rate is likely to be lower since the oblique components of the ®eld will have conversion coef®cients that are lower than that for 0. Except for two dosemeters with negligible response, the per-sonal dosemeter results agree within 50% with the reference values (Table 5). The results may improve once the dosemeter calibrations at the CANEL facil-ity are fully analysed using the MCNP-computed as well as the experimental energy and direction spectra. For the measurement point under the reactor (SAR), the reference values of Hp(10) cannot be deduced from the Bonner-sphere measurements because the irradiation ®eld was coming from the top and from room scattering on several structures surrounding the measurement point. In this case, direction spectrometry is needed to assess the beha-viour of the personal dosemeters.
CONCLUSIONS
The EVIDOS programme has currently reached its mid-term. One calibration campaign and two workplace campaigns have been performed. A large
quantity of data has already been collected and data are being analysed. Preliminary results show reason-able behaviour of several radiation protection instru-ments, in particular the area monitors but also several novel personal dosemeters. But they also highlight the well-known necessity to apply, for sev-eral instruments, a ®eld-speci®c correction factor to derive suf®ciently accurate results in workplace ®elds. Knowledge of the direction and energy distri-bution of the irradiation ®eld is essential to under-stand and improve the response of the instruments used. To improve the response of personal dose-meters, a well-characterised (in energy and direction) workplace-simulated reference ®eld is needed.
Table 5. Personal dose equivalent response of personal dosemeters, RHp Hp(10)measured/Hp(10)reference and relative standard uncertainties measured at KruÈmmel, cask
midline.
Name of device RHp u(RHp)/RHp
Aloka PDM-313 0.96 31
BTI-PND 1.46 34
HpSLAB 1.35 33
Local device (TLD albedo) 1.39 54
PADC (NRPB) 0.47 36 PND BDT 1.46 33 PSI CR-39 0.31 >59 PSI DISN 1.25% (2 mm) 0.42 43 PSI DISN 4% (1 mm) 0.08 90 PTB DOS-2002 0.57 36 Saphydose-n 0.62 34 Siemens EPD N 0.02 115 Siemens EPD N2 0.22 35
The personal dosemeter readings are compared with Hp(10) obtained from the Bonner sphere measurement and assuming a broad, parallel incidence of neutrons. Table 4. Ambient dose equivalent response of area monitors: RH* H*(10)measured/H*(10)reference and relative standard
uncertainties measured at two workplaces in KruÈmmel.
Name of device Workplace
Cask midline Reactor SAR
RH u Rÿ H=RH (%) RH u Rÿ H=RH (%) BAE SYSTEMS 0.71 16 0.92 16 Berthold LB 6411 0.91 16 1.30 16 Harwell Leake N91 0.91 16 1.30 16 SSI Sievert B 0.84 22 1.28 22 Studsvik 2200's 0.79 16 1.09 16 Wendi-2 0.93 16 1.79 18
The reference values are derived from the Bonner-sphere measurements of the neutron energy distribution. The Sievert instrument used the following approximate relation between quality factor, Q, and dose mean lineal energy, yD: Q 0.52 0.28yD.
ACKNOWLEDGEMENTS
This research is partly funded by the European Com-mission under the auspices of the Euratom 5th Framework Programme: Nuclear Energy, 1998± 2002, Contract No. FIKR-CT-2001-00175. The authors wish to express their gratitude to the techni-cal and scienti®c staff at Cadarache and KruÈmmel who helped during the campaigns, particularly to Moaad Bakali, J. L. Pochat, L. Van Ryckeghem, J. F. Guerre-Chaley, H. Muller, B. Aselineau, C. ItieÂ, T. Lahaye, Q. Chau, P. Gerdes, G. Hallfarth, U. Schulz and U. Welte.
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